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System Description

The WRS to be installed at the Sustainablity Base at NASA Ames Research Center uses osmosis for generating clean water from waste water. The WRS is mainly comprised of a forward osmosis (FO) module and a reverse osmosis (RO) module. FO is the movement of solvent molecules across a semi-permeable membrane from a region of higher water chemical potential (usually called the feed solution) to a region of lower water chemical potential (usually called the osmotic agent). RO, on the other hand, is the movement of solvent molecules across a semi-permeable membrane in the opposite direction of FO, i.e., from a region of lower water chemical potential to a region of higher water chemical potential due to the application of hydraulic pressure. In the FO system of the WRS, the waste water is the feed solution, and salt water is used as the osmotic agent. In the RO system of the WRS, hydraulic pressure is applied to the (now dilute) saltwater to extract clean water.

WRSSchematicFull
Figure 2: Schematic of the Water Recycling System

The figure above presents a schematic of the WRS, which consists of several tanks, pumps, pipes, filters, and the FO and RO modules. During nominal operation the WRS goes through the following steps:

  • Pump 1 is switched on to pump water from the Waste Water Tank into Feed Tank 1 till the latter is full.

  • Pump 1 shuts off and Pump 2 is turned on to fill Feed Tank 2. Filter 1 between Pump 2 and Feed Tank 2 traps suspended solids in the feed solution and prevents them from entering Feed Tank 2. Pump 2 runs till Feed Tank 2 is full. Pumps 5 and 6 are small diaphragm metering pumps that are turned on periodically to add antiscale chemicals (from the Antiscale Supply Tank) to the feed and adjust its pH (by adding chemicals from the pH Adjust Tank), respectively.

  • Then Pump 4 is powered on to recirculate the feed water through Filter 2, and the FO module back to the Feed Tank 2. The osmotic agent is stored in the Osmotic Agent (OA) Tank. The OA in the WRS is a salt (NaCl) solution. The concentration of OA determines the rate of flow of water. The goal is to maintain this flow at approximately 155 Lh-1. However, during the nominal operation of the WRS, some NaCl is lost through the membranes. Hence, additional NaCl is added to the OA to maintain the flow of water through the membrane. The initial concentration of OA is 10 gL-1, but the controller can add up to 20 gL-1 of additional NaCl solution to the OA from the NaCl Supply Tank. The RO module applies an external pressure to maintain the flow of water through the RO membrane to approx 155 Lh-1. The Reverse Osmosis (RO) pump recirculates the diluted OA between the RO and the FO modules. Clean water from the RO Module is collected in the Product Tank.

  • The WRS is operated in a semi-batch mode, with no extra feed added to Feed Tank 2 once the FO and RO modules are started till 95%

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